The fundamental characteristic of sickle cell disease is that it is a disease of abnormal blood rheology. A variety of basic hypotheses have been suggested to explain the major components of clinical severity in sickle cell patients. These hypotheses include: (1) the thermodynamic (equilibrium) hypothesis, (2) the kinetic hypothesis, (3) the adhesion hypothesis, and (4) the morphology hypothesis. All of these hypotheses bear on the rheology of sickle blood. Cells that have more polymer, that polymerize more rapidly, that adhere more readily, or that are misshapen and rigid will cause a higher suspension viscosity. In actuality, these major hypotheses are interdependent. However, they differ in that they attribute the primary pathogenic effects to the intracellular sickle hemoglobin polymer (at steady state or as kinetically determined) or to the membrane (directly in irreversibly deformed cells or through interactions with the endothelium in reversibly polymerized cells). The objectives of the proposed work are to measure rheological and ultrasonic parameters related to these hypotheses during deoxygenation of sickle cells so as to test the relevance of these hypotheses physiologically, and to use these methods to develop means to evaluate potential therapies for this disease. In particular we have three specific aims: (1) to continue with our studies of the rheology of packed cell suspensions and hemoglobin solutions. Of special interest is the rheological behavior at different oxygen saturations for a system "at equilibrium" and during time-dependent polymerization. The use of ultrasonic properties as a means to follow and characterize the rate and extent of polymerization will be examined; (2) to quantify the interactions of sickle cells with endothelial cells or protein-coated surfaces as polymerization of sickle cells occurs upon changes in environmental conditions; and (3) to determine the effects of pharmacological agents on the kinetics and extent of hemoglobin gel formation, and on the rheological and ultrasonic properties of sickle cells. All of these studies will be done with a unique microrheometer developed in our laboratory that can provide both ultrasonic and rheological information. In addition, electron microscopy will be used to correlate the state of hemoglobin within the cells (using TEM) to their rheological and ultrasonic properties and to their shape (determined using SEM). These studies should provide a better understanding of the intracellular gelation of sickle hemoglobin, provide a quantitative assay for intracellular polymerization, and form the basis for a better description of the pathophysiology of sickle cell anemia and evaluation of new therapeutic agents.